I just upgraded to a 2600K from my E8400. The rig is primarily used for gaming more than anything else, but i've noticed huge gains in all that I've been doing and am pleased. It wasn't just the CPU that was upgraded though, it was the entire setup.

Just think of it this way - if you feel the need to ask if you SHOULD upgrade, chances are you are pleased with your current performance and don't really need to. If you WANT to.. well then that's another story entirely.

I'd say it will be an upgrade, especially for games that use more than 2 cores, like bf:bc 2 and black ops. Those games will see a HUGE perfomance increase.
The real question is: is it worth it? That is what im debating about in my head :D

Ya, I love playing the newest games that come out. And Moving to a quad would make some sense. I am sure the i7 2600K or i5 2500K would last me about 3 years. Then again. Waiting for another year for the other chips to come out. Man It's a hard decision right now.